A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.
The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses extreme ultraviolet (EUV) radiation, being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm). A free electron laser may be used to generate EUV radiation for use by a lithographic apparatus.
It may be desirable to control the power of radiation output by a free electron laser. It may be desirable to control the power of free electron laser generated radiation received by a lithographic apparatus.
It is an object of the present invention to obviate or mitigate at least one problem associated with the prior art.